The present invention relates to a conductive resin composition and process for the preparation thereof, and more particularly, to a composition and mixing process that yields a conductive coating.
In the construction of metal bodies, such as the metal bodies used by the automotive industry, bare metal sections, which ultimately form the automobile body, are welded together typically by spot welding or mig welding. Such welding inherently causes ripples, porosity and other deformations along the weld seam as a result of the welding operation.
There have been a number of previously known methods for obscuring or hiding the imperfections caused by the weld seam. Such imperfections must, of course, be minimized prior to painting the body. Otherwise, the imperfections will show through the finish and detract from the overall appearance of the finish.
One previously known method of hiding the welded joint in the metal body is to apply solder to the joint and then subsequently grind the joint to form a flush surface between the two body panels. Such soldering, however, is disadvantageous since it is time consuming and also economically unfriendly due to the toxicity of lead in the solder. Although current solders do not contain lead, they do not finish as well as solders containing lead.
A more modem approach to obscuring the weld seam between two metal body panels is known as applying conductive resin composition.
A conductive resin composition includes between 40 and 140 parts by weight of a reactive epoxy resin, between 15 and 40 parts by weight of a reactive monofunctional glycidyl material, a cure accelerant, and 40 to 200 parts by weight of a conductive particulate material. The reactive epoxy resin typically including solid particulate of the cure accelerant that upon heating melts to cure. The reactive epoxy resin is preferably a one part curable resin optionally containing fillers, pigments, and stabilizers. A process for preparing a homogeneous conductive resin paste includes the steps of mixing from 50 to 150 parts by weight of a one-part epoxy resin composition under vacuum and at a temperature insufficient to induce thermal cure. The one-part epoxy resin composition including a reactive epoxy resin, a reactive monofunctional glycidyl material and a cure accelerant. Thereafter, 1 to 10 parts by weight of a liquid epoxy resin are added to form a mixture that is combined with 40 to 200 parts by weight of conductive particulate. The combined conductive particulate and the mixture are dispersed under vacuum until a homogeneous conductive resin paste is reached. The dispersal time during which the conductive particulate and the mixture are dispersed is a time greater than or equal to the mixing time for the one part epoxy resin composition. The resulting homogeneous conductive resin paste thermally cures absent of defects. The homogeneous conductive resin paste being deposited onto a metal substrate and heated to induce cure of the conductive resin paste to form a defect-free epoxy coating.
The present invention has utility as a protective coating that is electrically conductive and sufficiently uniform to yield a paintable surface. The inventive composition achieves a gloss coating of greater than 75 after a production paint overcoating. An inventive conductive resin composition includes an epoxy resin component present from between 40 and 150 parts by weight. Preferably, the epoxy resin component is present from 80 to 110 parts by weight. Preferably, the epoxy resin within the epoxy resin component is present from 60 to 80 parts by weight. Epoxy resins are detailed in the Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 9, pages 267-289, 3rd Edition. Epoxy resins operative in the present invention illustratively include the diglycidyl ether of bisphenol A, epoxy phenol novolak resins, tetraglycidyl ether of tetrakis (4-hydroxphenyl) ethane; N, N, Nxe2x80x2, Nxe2x80x2-tetraglycidyl-4,4xe2x80x2 diaminodiphenylmethane; triglycidyl isocyanurate, triglycidyl-paminophenol, diglycidyl ether of butane diol; 3,4-epoxycyclohexyloxirane, epoxycresol novolak (ECN) resins; resins derived from bisphenol A; epoxidized natural oils including epoxidized soybean oil; derivatives of tetraglycidyl methylene dianiline; derivatives of triazines such as triglycidyl isocyanurate and resins derived from the reaction of epichlorohydrin and a polyglycol.
An inventive conductive resin composition also includes between 15 and 40 parts by weight of a reactive monofunctional glycidyl material. The glycidyl material preferably being a monofunctional epoxy compound. More preferably, the monofunctional epoxy compound is an aliphatic glycidyl ether. Still more preferably, the aliphatic glycidyl ether has an aliphatic chain containing between 4 and 18 carbon atoms. Reactive glycidyl materials illustratively include butyl glycidyl ether, phenyl glycidyl ether and glycidyl methacrylate. It is appreciated that additional monofunctional epoxy compounds to those illustratively detailed herein are also operative. Preferably, the reactive glycidyl material is present from 15 to 25 weight percent.
A cure accelerant is also included within a conductive resin composition for inducing cure of the epoxy resin component and the reactive glycidyl material component. The cure accelerant used in the inventive conductive resin compositions is a compound which is an insoluble solid in the epoxy resin at room temperature yet is solubilized through heating to serve as a cure accelerant. A cure accelerant according to the present invention illustratively includes imidazole compounds that are solids at 25xc2x0 C., and imidazole adduct materials produced through the reaction of imidazole compound with an epoxy species, or a solid imidazole adduct created by the reaction of an imidazole compound with an isocyanate or urea species. Typically, a cure accelerant is present from between 0.05 and 2 parts by weight of the total weight of an inventive conductive resin composition. It is appreciated that the amount of cure accelerant present is dependent upon factors illustratively including the molecular weight of the accelerant, the uncured resin composition, the cure accelerant particle size, cure accelerant solubilization temperature and epoxy resin solvent polarity.
An additional component of the inventive conductive resin composition is an electrically conductive particulate material. Preferably, the electrically conductive particulate material is present from 40 to 200 parts by weight. The conductive particulate material operative herein illustratively includes particulate of graphite, copper, silver, aluminum, iron, magnesium, turbostratic carbon, and alloys thereof. More preferably, the conductive particulate is present from 50 to 70 parts by weight. Where carbon particulate is present, it is appreciated that the particulate shape is largely immaterial and can include granular, spherical, dendritic, flake, irregular shapes or mixtures thereof. The load fraction of conductive particulate being dictated to exceed the percolation threshold thus allowing an electrical charge to traverse a portion of the cured resin through contacting particles.
The present invention also relates to a process for preparing a homogeneous conductive resin paste capable of curing to a gloss measurement of greater than 75 gloss units after a production paint overcoating. Gloss being measured with an Electrometer Model 405 gloss meter (Manchester, UK) operating on a 0-100 scale.
A process for preparing an inventive homogenous conductive resin paste includes mixing a one part epoxy resin composition including an epoxy resin, a reactive glycidyl material and a cure accelerant suitable for inducing thermal cure between the epoxy resin and glycidyl material components. The one part epoxy resin composition optionally contains fillers, pigments, stabilizers, or similar materials. The one part epoxy resin composition is mixed under vacuum conditions until gas bubbles are removed. While mixing times vary depending upon the size of the cure accelerant particulate and batch size, 1 to 6 hours is a typical time range to adequately degas the one part epoxy resin composition. Thereafter, 1 to 10 parts by weight of a liquid epoxy resin is optionally added to the degassed mixture in order to modify the viscosity of the uncured resin and/or to modify the flexibility characteristics of the resulting cured coating. Preferably, the liquid epoxy resin when present is added at 2 to 5 parts by weight. Ideally, no cure accelerants are included in the liquid epoxy resin, nor are there other forms of particulate dispersed therein. The resulting mixture of one part epoxy resin composition and liquid epoxy resin is stirred under vacuum until homogeneous and degassed as detailed above. It is appreciated that heat is associated with mechanical mixing, a cooling jacket is often required to prevent premature resin curing. Typically, the mixture of one part epoxy resin composition and liquid epoxy resin is mixed under vacuum for an additional thirty minutes to three hours until degassed and homogenized. The resulting mixture is combined with 40 to 200 parts by weight of conductive particulate and further mixed under vacuum until the mixture wets the conductive particulate. Upon fully mixing the conductive particulate into the mixture, a paste results that is suitable to application to a substrate. The substrate illustratively includes metal sheeting, especially those used in the construction of vehicle outer body panels such as cold-rolled steel, galvanized steel and aluminum. The inventive conductive resin composition paste is deposited in paste form, onto the metal surface. Heating of the metal surface to a temperature of about 300 to 400xc2x0 F. is sufficient to cross-link the resin composition.